The present invention relates generally to a telephoto lens, a telephoto lens system and an imaging system incorporating the same, and more particularly to a telephoto lens of the inner focus type best suited for use with digital cameras, a telephoto lens system best suited for use with digital cameras of the lens interchangeable type and an imaging system incorporating the same.
So far, quite a number of telephoto lenses of the inner focus type have been proposed as interchangeable lenses used on single-lens reflex cameras for silver halide photography, and various constructions have been put forward for such telephoto lenses wherein only inner lenses are moved for focusing from infinity to the nearest distance.
For instance, conventional telephoto lenses of patent publications 1 and 2 are each comprised of, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power and a third lens group having positive refracting power. For focusing, the second or the third lens group is moved. The first lens group having positive refracting power is made up of two positive lenses and one negative lens.
With the prior art telephoto lenses of patent publications 1 and 2, however, it is still difficult to maintain the integrity of performance at the nearest distance while axial chromatic aberrations are well corrected; the limits of the ability to maintain the integrity of performance are about 0.1 as expressed in terms of magnification.
Patent publication 3 shows a telephoto lens comprising, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having negative refracting power. This telephoto lens is designed to perform focusing through the second lens group having negative refracting power, and the fourth lens group having negative refracting power, with improvements added thereto in such a way as to maintain the integrity of performance at the nearest distance.
Even with this telephoto lens, however, there are still large fluctuations of spherical aberrations at near distances, with insufficient correction of distortion throughout the optical system.
Besides, there are optical systems such as those set forth in patent publications 4, 5, 6 and 7. All such optical systems comprise a first lens group having positive refracting power, a second lens group having negative refracting power and a third lens group having positive refracting power, wherein focusing is carried out by movement of the second lens group. In consideration of taking magnification, the nearest distance, aperture relative to image pickup planes, performance, etc. however, they are less than satisfactory especially for such digital camera purposes as mentioned just below.
With recent progresses in digital cameras, lens interchangeable digital cameras are now commercialized. Most such digital cameras are designed to use silver-halide optical systems as interchangeable lenses without any modification thereto.
Technical improvements in image pickup devices lead to digital cameras making use of an image pickup device having far more pixels. To address such high-definition image pickup devices, associated optical systems, too, are required to have ever higher resolution. When an optical system used with conventional silver halide single-lens reflex cameraa is built in a digital camera making use of such a high-definition image pickup device as a telephoto lens, sufficient resolving power can never be obtained.
With the emergence of new users enjoying digital cameras, there are ever greater demands for performance requirements. For instance, much more decreased distortion and chromatic aberrations as well as the integrity of performance at the nearest distance are now in demand. In this regard, too, there are problems with the application to telephoto lenses of optical systems so far used with single-lens reflex cameras.
Patent Publication 1                JP(A) 55-147606        
Patent Publication 2                JP(A) 2000-89103        
Patent Publication 3                JP(A) 61-215513        
Patent Publication 4                JP(A) 9-15991        
Patent Publication 5                JP(A) 2000-258685        
Patent Publication 6                U.S. Pat. No. 3,445,554        
Patent Publication 7                JP(A) 6-201988        